Precipitation-hardenable alloys such as the 7000 series aluminum alloys, are subjected typically to a series of precisely controlled thermal treatment steps to improve yield strength and corrosion resistance of the alloy. The mechanical and physical properties of the heat-treated alloy depend upon the relative amounts of each of a plurality of different precipitate phases that are formed during the heat treatment process. Often, the amount of each precipitate phase is expressed as a volume fraction. The 7000 series aluminum alloys are conventionally processed in the T6 temper condition (peak age) or T73 temper condition (overage). The T6 alloys usually contain predominantly meta-stable coherent precipitates and have high strength but poor resistance to stress corrosion cracking (SCC). The T73 alloys, on the other hand, contain large amounts of semi-coherent and incoherent precipitates and have good corrosion resistance but with a rather significant reduction in strength relative to that of the T6 alloys.
A treatment known as Retrogression and Reaging (RRA) can be applied to material in a T6 temper condition (solution treatment followed by a 24 hours of artificial aging at 120° C.) to yield material strength levels equivalent to the T6 material while also having corrosion resistance equivalent to the T73 condition. The RRA process consists of two steps: a) retrogression in the range 180-240° C., followed by water-quenching and b) reaging at about 120° C. for 24 hours. The retrogression step a) is a very critical step and must be controlled carefully. At the higher temperatures of 220-240° C. the optimum time for retrogression may be only a few minutes or even seconds, while at the lower temperatures of 180-200° C. the optimum time may be up to 60 minutes. Such a treatment can be used to obtain an optimised combination of strength and corrosion resistance in 7000 series alloys. RRA processing is of particular interest to aircraft operators, as the technology can be effectively applied to address issues of corrosion damage in ageing aircraft. The technology involves short time heat treatment of alloys in the T6 temper, followed by a re-ageing treatment, as result of which SCC resistance equivalent to that of the T73 temper is achieved with no significant penalty in strength relative to that of the T6 temper. Application of RRA to aircraft components, either by bulk treatment or localized heat treatment, requires tight control over the thermal exposure history during processing. Unfortunately, there are no quantitative criteria that can be used to assess the properties of the processed component after it has been processed according to some arbitrary thermal exposure profile. As such, the properties of the alloy component must be determined by post-treatment testing, which testing often is other than practical when dealing with aircraft components. To overcome such disadvantage, a simulation of the precipitation reactions that occur during RRA will be beneficial to optimise the process.
In “Kinetics for . . . Predicting the Effects of Heat Treating Precipitation-Hardenable Aluminum Alloys”, Industrial Heating, 44(10) 1977 pp. 6-9, J. T. Staley discloses a process which permits quantitative compensation of effects of precipitation on the yield strength of the material during heating and/or during soaking either above or below the recommended temperature. However, said process produces the metal in either the T6 or the T73 temper state after quenching, and does not address the kinetic issues when a combination of strength and corrosion resistance is to be considered.
Therefore, a problem and a challenge to designers is to predict the properties of a material based on any thermal exposure it experiences, which may then be used as criteria for assessing heat exposure effects, including the effects of heat treatments.